[UPDATE October 2016] I’ve been using this option pretty much daily ever since posting this in 2013. It’s worked fine in all of the browsers I’ve tried. But recently, in Apple’s Safari browser (all other browsers I’ve tried are fine), the sound muffles slightly when videos are speeded up. The speed boost still works in Safari. I sometimes use it anyway, but for best playback audio quality, browsers other than Safari are preferred. Browsers I’ve tried that have excellent sound quality when speed boosted are Firefox, Chrome and Opera.
Also, I recently posted this: HOW TO CHANGE THE PLAYBACK SPEED OF A YOUTUBE VIDEO (tutorial)
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From: TimeAtlas.com

How To Speed Up Videos

Speeding up Online Videos

Most times, I was trying to speed up videos that I was viewing on the web. In these instances, the best program I’ve found is Enounce MySpeed . It’s a small app that monitors online videos. If the tool detects that it can adjust the video speed, it displays a small slider. This slider allows you to adjust video playback speed within a set range from .3x to 5x. …

Speeding up YouTube Videos

If your primary goal is to speed up YouTube videos, there’s a free option in testing. You’ll have to sign up for the YouTube HTML5 trial and use a supported browser. You can see in the screen snap below, the YouTube interface is slightly different than in my previous example. The speed control is built into the bottom portion of the video frame. Clicking the Speed button opens a menu where you can increase the video playback from .25x to 2x.

As the speed feature is still in testing, it doesn’t work on all online videos. Some known limitations include full screen videos and videos with ads. There may be additional issues if you’re testing other YouTube features.

Using Windows Media Player to Speed Up Videos

Sometimes the video you want to speed up is a movie you’ve downloaded to your computer such as an .avi, .wmv or .mp4 file. In this case, I would use Windows Media Player enhancements. The free program is included with most versions of Microsoft Windows and you can download the newest version from their site . …

Using QuickTime Player to Speed Up Videos

Another popular video player is Apple’s free QuickTime video player that is bundled with iTunes. It plays .mov, .mp4, .avi and MPEG-1 files. Like Windows Media player, it also allows you to adjust the speed on videos you’ve downloaded to your PC.

Entire Article Here

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